Balance disorders and vertigo are a significant health problem in the US, and are a leading cause of injures from falling in the elderly. While there are many factors that contribute to disorders in balance, pathology in the vestibular sense organs is thought to be a major contributor. There is a progressive loss in hair cell number in the semicircular canal cristae and the maculae of people as they age. Functional testing shows a similar decline in peripheral vestibular function with increasing age. There are currently no effective treatments to restore peripheral vestibular function. Over the past 20 years, there have been several reports that suggest new hair cells can be generated in the vestibular sensory epithelia. Some of the more promising data come from studies of the cristae, particularly in the chinchilla;however, there is as yet, no definitive proof that new hair cells can be generated in the adult mammalian cristae. The basic problem is that the tools available for studies of hair cell regeneration in species like the chinchilla are fairly limited. Most of these studies have been carried out with traditional morphological analyses, which cannot adequately assess whether any recovery from ototoxic drugs is due to de novo hair cell regeneration, transdifferentiation from existing support cells, or repair of the damaged hair cells. In a recent study of Notch signaling in mouse inner ear, we found that while this developmental signaling pathway is no longer active in the mature cochlea, it appears to be active in the support cells of the cristae. We reasoned that if the Notch pathway was still active in the mature mouse cristae, we could stimulate hair cell replacement through transdifferentiation. In preliminary studies, with pharmacological regulators of the Notch pathway, we have found evidence that support cells can transdifferentiate into hair cells in mature mouse cristae. We propose to follow-up on these new findings using the extensive genetic tools available in the mouse to determine whether (1) Notch signaling remains active in the mature mammalian cristae (2) Support cells can be induced to transdifferentiate into hair cells through inhibition of the Notch pathway (3) The mature mammalian cristae can regenerate hair cells in vivo by de novo generation or transdifferentiation. We further propose to attempt to stimulate hair cell replacement in mature mouse cristae in vivo through manipulations of the Notch pathway, and carry out functional analysis to determine whether any morphological evidence for hair cell replacement correlates with functional recovery. At the present time there are no viable methods to restore hair cells in the vestibular epithelia as they are lost with aging. If we find that we can stimulate recovery in this system with pharmacological manipulations in the Notch pathway, this could offer benefit to millions of elderly individuals to improve their quality of life and decrease their risk of injury. PUBLIC HEALTH RELEVANCE: Balance disorders and vertigo are a significant health problem in the US, and are a leading cause of injures from falling in the elderly. While there are many factors that contribute to disorders in balance, pathology in the vestibular sense organs is thought to be a major contributor. If we find that we can stimulate recovery in this system with pharmacological manipulations in the Notch pathway, this could offer benefit to millions of elderly individuals to improve their quality of life and decrease their risk of injury.